This invention relates to improved control of an extrusion apparatus lip gap.
As exemplified by U.S. Pat. Nos. 5,208,047 and 5,423,668, having Peter F. Cloeren as a common inventor, an extrusion apparatus having an adjustment assembly for thermal and mechanical, localized adjustment of lip gap, is known. In the '047 patent, a cartridge heater is removably disposed within a bore in a push rod which extends to operatively contact the flex lip thereof, and which is constructed of a thermally responsive material for expansion and contraction and thereby for thermal adjustment of the lip gap. Also described in the '047 patent is a temperature-responsive push rod with a resistance heater wire coiled around in direct heat transfer contact. In either case, the thermal output is adjusted in response to thickness measurement of the film or sheet passing through the lip gap.
As described in the '047 and '668 patents, a lip adjustment screw having an end bearing upon a push rod for mechanical adjustment of the lip gap, is likewise known. The lip adjustment screw may be in threaded engagement with a nut disposed in a shoulder recess. Variations using a differential thread, adjustment screw are also known, for instance, a differential thread, adjustment screw in axial alignment with a push rod, rather than offset from the push rod axis, with coarser pitch threads in engagement with a nut disposed in a shoulder recess and with finer pitch threads in engagement with the push rod. Alternatively, the finer pitch threads may be in engagement with a second nut.
Again with regard to thermal adjustment of the lip gap, the state of the art as further illustrated by U.S. Pat. Nos. 3,940,221 to Nissel, 4,454,084 to Smith et al, and 4,753,587 to Djordjevic et al, is a push rod of significantly greater length than the removably mounted, cartridge heater. As a result, the heating response of the push rod is dependent upon a substantially shorter heat source. The push rod functions as a translator to convert heat energy to mechanical force exerted upon the lip, by expansion and contraction.
In the Nissel device, the push rod passes through a thermal transfer block in which the cartridge heater is disposed. In the Smith structure and with particular reference to FIG. 3, a threaded push rod has an internally disposed cartridge heater, which by scale is about 70% of the push rod length. The Djordjevic device shown in FIG. 1 is similar, and by scale, the internally disposed cartridge heater is about 50% of the push rod length.
Also commercially known is an extrusion apparatus having a cartridge heater/push rod combination in which the internally disposed, cartridge heater is about 60% of the push rod length. In this apparatus, the push rod does not operatively contact the die lip; instead, this push rod bears upon another push rod which does operatively contact the die lip.
With reference particularly to FIG. 4 of Cloeren '047, the push rod structure corresponding thereto in commercial use, has a length of 6 inches and the cartridge heater has a length of 4 inches. In this device, the cartridge heater is about 66% of the push rod length.
The relatively greater length increases the structural mass of the push rod. In addition, whereas an exteriorly disposed, resistance heater wire as in FIG. 3 of Cloeren '047, provides for direct heat transfer contact with a temperature-responsive push rod, a drawback with an interiorly disposed, removable cartridge heater is lack of efficient thermal energy transfer to the temperature-responsive push rod.
Therefore, there is a long felt need to improve the performance of a thermal energy translator having an interiorly disposed, thermal energy source. More particularly, there exists a need for a thermal energy source/translator combination having faster expansion response, improved heat transfer from the thermal energy source to the thermal energy translator, and a more beneficial balance between the length of the heat source and the length of the thermal energy translator. In addition, there is an over-arching need to improve energy efficiency in connection with thermal control of the lip gap.